1. Field of the Invention
The present invention relates to a display apparatus. More particularly, the invention relates to a display apparatus that comprises a number of electron-emitting elements.
2. Description of the Related Art
In recent years, light, thin planar display apparatuses have been developed as next-generation displays. They comprise a phosphor screen and a number of electron-emitting elements (hereinafter referred to as xe2x80x9cemittersxe2x80x9d) that oppose the phosphor screen. The emitters may be of the field-emission type or the surface-conduction type. Any display apparatus that comprises electron-emitting elements of field-emission type, used as emitters, is generally called xe2x80x9cfield emission display (hereinafter referred to as xe2x80x9cFEDxe2x80x9d). Any display apparatus that comprises electron-emitting elements of surface- conduction type, used as emitters, is called xe2x80x9csurface-conduction type, electron-emitting display (hereinafter referred to as xe2x80x9cSEDxe2x80x9d).
Generally, an FED has a face plate and a rear plate, which oppose each other and are spaced apart with a prescribed gap between them. The substrates are joined together at their peripheral edges, with a rectangular frame shaped side wall interposed between them. The substrates and the side wall constitute a vacuum envelope. A phosphor screen is formed on the inner surface of the face plate. A number of emitters are provided on the inner surface of the rear plate. The emitters are used as elements for emitting electrons that excite the phosphor, causing the phosphor to emit light. A plurality of support members are arranged between the rear plate and the face plate, preventing the substrates from collapsing due to the atmospheric pressure applied on the plates.
The rear plate is at a potential of about 0V. An anode voltage Va is applied to the phosphor screen. The electron beams emitted by the emitters are applied to the red, green and blue phosphors of the phosphor screen. Upon receiving the electrons, the phosphors emit light, whereby the FED displays an image.
In the FED, the gap between the front and rear plates can be reduced to a few millimeters or less. The FED can therefore be lighter and thinner than cathode-ray tubes (CRTs) that are used at present as TV displays and computer displays.
With the display apparatus thus structured, it is necessary to use phosphors of the same type as used in ordinary cathode-ray tubes and to set the anode voltage at several kilovolts or more, so that the apparatus may acquire practically useful characteristics. However, the gap between the front and rear plates cannot be so large, in view of the resolution, the characteristics of support members, the manufacturing ease, and the like. The gap should be about 1 to 2 mm. An intense electric field will inevitably develop between the front and rear plates, and discharge (dielectric breakdown) may occur between the plates.
If discharge takes place, the emitters and the phosphor screen may be broken or deteriorated. Discharge should not occur in the product because it would result in errors. Nonetheless, it is extremely difficult to prevent the discharge.
The discharge may be controlled, not prevented, so that the influence it imposes on the emitters may be negligibly small. This technical concept is similar to the technical concept which is widely applied in the field of CRTs and known as xe2x80x9csoft flashing.xe2x80x9d This technique is to increase the resistance of the film on the inner surface of a CRT to reduce the discharge current. Thus, the technique can prevents the breakdown of the circuit incorporated in the tube even if discharge takes place.
In the FED and the SED, however, the phosphor screen acts as a discharge electrode, and thus, the above-mentioned technique cannot be employed, without any countermeasures.
The present invention has been made in view of the foregoing, and its object is to provide a display apparatus in which, if discharge occurs, the discharge current can be controlled to prevent the emitters and phosphor screen from being broken or deteriorated.
To attain the object, a display apparatus according to an aspect of the invention comprises: a face plate having a phosphor screen formed on an inner surface of the face plate; a rear plate opposing the phosphor screen and having a plurality of electron-emitting elements which emit electrons toward the phosphor screen; a transparent insulating substrate opposing an outer surface of the face plate; and a resistive layer provided between the face plate and the insulating substrate.
In the display apparatus according to the aspect of the invention, it is desired that the resistive layer has a sheet resistance of 10 xcexa9/xe2x96xa1 or more, and the resistive layer may comprise a transparent conductive film or may be formed of filler or the like.
In the display apparatus thus structured, the insulating substrate opposes the outer surface of the face plate and the anode voltage or a similar voltage is applied to the outer surface of the face plate, too. This can minimize the charge accumulated in the face plate, almost to zero. The insulating substrate indeed accumulates an electric charge. However, this charge cannot reach the discharging section unless it passes through the resistive layer, because the resistive layer is provided between the face plate and the reinforced glass plate. Hence, the discharge current can be controlled to prevent the emitters and phosphor screen from being broken and deteriorated.
Assume discharge occurs between the face plate and the rear plate. The magnitude of this discharge is determined by the charge accumulated in the capacitor comprising the front and rear plates. The capacitor is constituted by a capacitor C1 provided between the front and rear plate and a capacitor C2 defined between the inner and outer surfaces of the face plate. The capacitors C1 and C2 can be regarded as being connected in parallel to each other. If an aspect of the present invention is not applied, the voltage at the face plate will instantaneously become almost 0V. If this happens, most charge accumulated in C1 and C2 will become a discharge current.
In the display apparatus according to the embodiment of the invention, a potential difference between the inner and outer surfaces of the face plate is rendered zero, and C2 will generate no charge. Generally, C2 is far greater than C1 because a glass layer having permittivity of about 8 is inserted in C2. In order to make the apparatus light, it is desirable to reduce the thickness of the face plate. If the face plate is thin, however, C2 will increases. In view of this, it is very advantageous that the influence of C2 can be eliminated. Although the application of this invention cannot completely eliminate the influence of C1, the magnitude of discharge will greatly decrease. This is because C2 is much greater than C1.